iMessage is extraordinarily popular in the US. Its userbase dwarfs Signal by over an order of magnitude
A year ago someone on HN said “I can confirm that iMessage is extremely common in Australia. WhatsApp is very uncommon, outside of people with European (and maybe South American?) friends or family to keep in touch with.”
https://news.ycombinator.com/item?id=39365562
My guess is you’re both expressing truths of your individual social circles but making unjustified extrapolations to an entire nation.
The issue with iMessage outside the US is the branding, it's branded as an SMS app and SMS being dead (outside of ads and delivery drivers) doesn't help for adoption.
This has more to do with the way the iPhone was launched, and the American desire to own the most expensive product, than any technical merits.